Antenna configuration

ABSTRACT

Power is transmitted to an implanted receiving coil oriented such that an axis of the receiving coil is parallel to skin of a subject. A transmitting coil is in a housing, which is placed against the skin. A central axis of the transmitting coil is perpendicular to the skin. A portion of the transmitting coil is over the receiving coil. A first distance, from the axis of the transmitting coil to a center of the receiving coil, is greater than a second distance, from the axis of the transmitting coil to an inner edge of the portion of the transmitting coil. The first distance is less than a third distance, from the axis of the transmitting coil to an outer edge of the portion of the transmitting coil. Circuitry powers the implant by driving current through the transmitting coil that induces current in the receiving coil.

FIELD OF THE INVENTION

Applications of the present invention relate to transmitting power to animplanted medical device.

BACKGROUND

Electrical power can be transferred to a percutaneous medical implant bymagnetic induction. A current flowing through a coil produces a magneticfield, which, in turn, will induce a current in a second coil, providedthe second coil is in close enough proximity to the magnetic field andoriented such that the magnetic field is substantially parallel to thecentral longitudinal axis of the second coil. A coil inside a medicalimplant can therefore act as a receiving coil, while a coil outside apatient's body can act as a transmitting coil. A current can be driventhrough the transmitting coil in order to induce an induced current inthe receiving coil, thereby powering the medical implant.

A CBS News article entitled “Migraine ‘smart’ patch tested to help easepain,” by Steven Reinberg, describes a study performed at Rambam MedicalCenter in Haifa, Israel, under Dr. David Yarnitzky, chair of neurologyat the Rambam Medical Center. The study tested an arm patch to be wornon the upper arm with “[r] ubber electrodes and a chip in the patch [to]produce electric impulses that block pain signals from reaching thebrain,” in order to treat migraine pain.

A St. Jude Medical, Inc. fact sheet entitled “Peripheral nervestimulation for intractable chronic migraine,” describes peripheralnerve stimulation as a treatment for chronic migraines. The fact sheetstates that “Peripheral nerve stimulation (PNS) is a therapy that usesmild electrical pulses to stimulate the nerves of the peripheral nervoussystem. The peripheral nerves make up a network of nerves outside of thecentral nervous system. For example, the ulnar nerve in the arms and thesciatic nerve in the legs are part of the peripheral nervous system. TheSt. Jude Medical systems currently approved for PNS in select marketslook and operate much like a cardiac pacemaker. However, instead ofsending pulses to the heart, the pulses are carried to the occipitalnerves, located in the back of the head. . . . Researchers believe thatby delivering electrical pulses to these specific peripheral nervefibers, PNS may influence the way the nerves communicate with the brainand provide an alternative to long-term drug therapy for the relief ofchronic migraine.”

SUMMARY OF THE INVENTION

A method is described for transmitting power to a medical implant thatincludes a receiving coil. For some applications, a transmitting coil,disposed in a transmitting coil housing, is placed against skin of asubject such that a central longitudinal axis of the transmitting coilis substantially perpendicular to the skin. For some applications, themedical implant is implanted between an ankle and a knee of a leg of asubject, typically closer to the ankle than the knee. To increaseefficiency of the power transfer while accommodating for limited spacenear the ankle, the transmitting coil is oriented with respect to theskin such that it is not centered over the receiving coil, but ratheronly a portion of the transmitting coil is disposed directly over thereceiving coil. This orientation of the transmitting coil with respectto the receiving coil allows for powering the medical implant using onlyone transmitting coil. To transmit power to the medical implant, controlcircuitry is activated to drive a current through the transmitting coilthat induces an induced current in the receiving coil.

Typically, the transmitting coil housing and the transmitting coil areflexible in order to comfortably conform to the shape of a limb of thesubject. In the absence of any forces applied to the transmitting coil,the transmitting coil has a nominal resonance frequency. In order toaccommodate for possible fluctuations in the resonance frequency of thetransmitting coil due to the flexing, a sensor may be coupled to thecontrol circuitry and configured to determine an extent of divergence of(a) the resonance frequency of the transmitting coil when thetransmitting coil is flexed from (b) the nominal resonance frequency ofthe transmitting coil, occurring in the absence of any forces applied tothe transmitting coil. The control circuitry is further configured tooutput a signal that controls one or more electrical components that are(a) coupled to the control circuitry and (b) configured to tune theresonance frequency of the transmitting coil in order to compensate forthe fluctuations.

There is therefore provided, in accordance with some applications of thepresent invention, a method for transmitting power to a medical implantthat includes a receiving coil that is oriented such that a longitudinalaxis of the receiving coil is substantially parallel to skin of asubject, the method including:

-   -   providing a transmitting coil disposed in a housing;    -   placing the housing against the skin such that:    -   (a) a central longitudinal axis of the transmitting coil is        substantially perpendicular to the skin,    -   (b) a portion of the transmitting coil is disposed over the        receiving coil,    -   (c) a first distance, from the central longitudinal axis of the        transmitting coil to a longitudinal center of the receiving        coil, is greater than a second distance, from the central        longitudinal axis of the transmitting coil to an inner edge of        the portion of the transmitting coil, and    -   (d) the first distance is less than a third distance, from the        central longitudinal axis of the transmitting coil to an outer        edge of the portion of the transmitting coil; and        activating control circuitry to power the medical implant by        driving a current through the transmitting coil that induces an        induced current in the receiving coil.

For some applications, placing includes identifying the subject assuffering from migraines or cluster headaches, and in response to theidentifying, placing the housing on a leg of a subject such that:

(a) the transmitting coil is disposed between a knee and an angle of theleg, and

(b) the transmitting coil transmits power to a medical implantconfigured to stimulate a tibial nerve in the leg of the subject.

For some applications, placing includes placing the housing on a leg ofthe subject such that:

(a) the transmitting coil is disposed between a knee and an ankle of theleg, and

(b) (i) a portion of the transmitting coil that is disposed over thereceiving coil is closer to the ankle than (ii) a portion of thetransmitting coil that is disposed at 180 degrees from the portion ofthe transmitting coil that is disposed over the receiving coil, is tothe ankle.

For some applications, placing includes placing the housing such thatthe first distance is 15-4.5 mm.

For some applications, placing includes placing the housing such thatthe second distance is less than 30 mm.

For some applications, placing includes placing the housing such thatthe third distance is 40-60 mm.

For some applications, placing includes placing the housing such that adifference between the third distance and the second distance is 30-40mm.

For some applications, providing the transmitting coil includesproviding a transmitting coil wherein a ratio of (a) a differencebetween the third distance and the second distance, to (b) alongitudinal length of the receiving coil is greater than 0.5.

For some applications, providing the transmitting coil includesproviding a transmitting coil wherein a ratio of (a) a differencebetween the third distance and the second distance, to (b) alongitudinal length of the receiving coil is less than 1.5.

For some applications, providing the transmitting coil includesproviding a transmitting coil wherein a ratio of (a) a differencebetween the third distance and the second distance, to (b) alongitudinal length of the receiving coil is between 0.5 and 1.5.

For some applications, providing the transmitting coil includesproviding a transmitting coil wherein:

(a) a height of the transmitting coil measured along a longitudinal axisof the transmitting coil is 300-600 microns,

(b) an outer diameter of the transmitting coil is 100-140 mm, and

(c) a ratio of the outer diameter of the transmitting coil to the heightof the transmitting coil is at least 150.

For some applications, placing includes placing the housing such thatthe transmitting coil is over a receiving coil, wherein:

(a) a longitudinal length of the receiving coil is 3-15 mm.

(b) an outer diameter of the receiving coil is 0.6-1.5 mm, and

(c) a ratio of the outer diameter of the receiving coil to thelongitudinal length of the receiving coil is less than 0.5.

For some applications, activating the control circuitry includesactivating the control circuitry to drive the current through thetransmitting coil at a frequency of 1-20 MHz.

For some applications, placing includes placing the housing against theskin and subsequently sliding it along the skin until an indicator,coupled to the housing, indicates that the transmitting coil is in anacceptable position with respect to the receiving coil.

For some applications, providing the transmitting coil includesproviding a transmitting coil wherein a cross-sectional area of a wireof the transmitting coil is rectangular, wherein the cross-section istaken perpendicular to a direction of current flow within the wire.

For some applications, providing the transmitting coil includesproviding a transmitting coil that is elongated in a directionperpendicular to the central longitudinal axis of the receiving coil.

For some applications, providing the transmitting coil includesproviding a planar coil disposed in a housing.

For some applications, providing the planar coil includes providing aplanar coil including a plurality of layers.

For some applications, providing the planar coil includes providing aplanar coil with a line spacing, of adjacent coplanar wires, of 0.25-3mm.

For some applications, providing the planar coil includes providing aplanar coil with a line width of 1-4 mm.

For some applications, providing the transmitting coil includesproviding a transmitting coil wherein an average distance from a wire ofthe transmitting coil to the central longitudinal axis of thetransmitting coil is less than two times a square root of across-sectional area of a central non-coiled region of the transmittingcoil.

For some applications, providing includes providing a transmitting coilwherein an average distance from the wire of the transmitting coil tothe central longitudinal axis of the transmitting coil is 0.6-1.5 timesthe square root of the cross-sectional area of the central non-coiledregion of the transmitting coil.

There is further provided, in accordance with some applications of thepresent invention, apparatus including:

a medical implant, the medical implant including:

-   -   a receiving coil; and    -   a plurality of electrodes;

a transmitting coil, having wire disposed at all rotational locationsabout a central longitudinal axis of the transmitting coil, orientedsuch that:

-   -   (a) the central longitudinal axis of the transmitting coil is        substantially perpendicular to a central longitudinal axis of        the receiving coil,    -   (b) at one of the rotational locations, a line from the wire and        substantially parallel to the central longitudinal axis of the        transmitting coil intersects the receiving coil, and at 180        degrees from the rotational location a line from the wire and        substantially parallel to the central longitudinal axis of the        transmitting coil does not intersect the receiving coil,    -   (c) a first distance from the central longitudinal axis of the        transmitting coil to a longitudinal center of the receiving        coil, is greater than a second distance from the central        longitudinal axis of the transmitting coil to an inner edge of        the transmitting coil at the one of the rotational locations,        and    -   (d) the first distance is less than a third distance from the        central longitudinal axis of the transmitting coil to an outer        edge of the transmitting coil at the one of the rotational        locations; and

control circuitry configured to transmit power to the medical implant bydriving a current through the transmitting coil that induces an inducedcurrent in the receiving coil.

For some applications, the control circuitry is configured to drive thecurrent through the transmitting coil at a frequency of 1-20 MHz.

For some applications, the medical implant is configured to be implanted1-5 cm below skin of a subject, and the control circuitry is configuredto transmit the power, by driving the current through the transmittingcoil that induces the induced current in the receiving coil, when themedical implant is implanted 1-5 cm below the skin.

For some applications, the receiving coil is a cylindrical coilincluding a ferrite core.

For some applications, the first distance is 15-45 mm.

For some applications, the second distance is less than 30 mm.

For some applications, the third distance is 40-60 mm.

For some applications, a difference between the third distance and thesecond distance is 30-40 mm.

For some applications, a ratio of (a) a difference between the thirddistance and the second distance, to (b) a longitudinal length of thereceiving coil is greater than 0.5.

For some applications, a ratio of (a) a difference between the thirddistance and the second distance, to (b) a longitudinal length of thereceiving coil is less than 1.5.

For some applications, a ratio of (a) a difference between the thirddistance and the second distance, to (b) a longitudinal length of thereceiving coil is between 0.5 and 1.5.

For some applications:

(a) a height of the transmitting coil measured along a longitudinal axisof the transmitting coil is 300-600 microns,

(b) an outer diameter of the transmitting coil is 100-140 mm, and

(c) a ratio of the outer diameter of the transmitting coil to the heightof the transmitting coil is at least 150.

For some applications:

(a) a longitudinal length of the receiving coil is 3-15 mm,

(b) an outer diameter of the receiving coil is 0.6-1.5 mm, and

(c) a ratio of the outer diameter of the receiving coil to thelongitudinal length of the receiving coil is less than 0.5.

For some applications:

(a) a first ratio, of the outer diameter of the transmitting coil to aheight of the transmitting coil measured along a longitudinal axis ofthe transmitting coil, is at least 150,

(b) a second ratio, of the outer diameter of the receiving coil to thelongitudinal length of the receiving coil, is less than 0.5, and

(c) a ratio of the first ratio to the second ratio is at least 300.

For some applications, the transmitting coil has between 4 and 10 turns.

For some applications, the receiving coil has between 10 and 40 turns.

For some applications, the medical implant is configured to send asignal to the control circuitry upon receiving the transmitted power.

For some applications, a cross-sectional area of a wire of thetransmitting coil is rectangular, the cross-section being takenperpendicular to a direction of current flow within the wire.

For some applications, the transmitting coil is elongated in a directionperpendicular to the central longitudinal axis of the receiving coil.

For some applications, a length of the receiving coil is 3-15 mm.

For some applications, the medical implant includes a housing having alength of 30-45 mm and the receiving coil is disposed in within thehousing.

For some applications, the apparatus further includes an indicator, andthe control circuitry is configured to activate the indicator upon thetransmitting coil being in an acceptable position with respect to thereceiving coil.

For some applications, the control circuitry is configured to detectinterference with its output signal and to activate the indicator uponthe detection of the interference.

For some applications, the control circuitry is configured to activatethe indicator again, upon the transmitting coil no longer being incorrect position with respect to the receiving coil.

For some applications, the control circuitry is configured to ascertainan indication of an efficiency of the power transfer between thetransmitting coil and the receiving coil, and to activate the indicatoraccording to the ascertaining.

For some applications, the control circuitry is configured to measure aloss of power in the transmitting coil, the loss of power beingindicative of the efficiency of the power transfer.

For some applications, the transmitting coil is a planar coil.

For some applications, a line width of the transmitting coil is 1-4 mm.

For some applications, the planar coil includes a plurality of layers.

For some applications, a line spacing of adjacent coplanar wires in thetransmitting coil is 0.25-3 mm.

For some applications, the apparatus further includes a flexible printedcircuit board (PCB), and the transmitting coil includes two planarlayers disposed on either side of the flexible PCB.

For some applications, a height of each layer measured along alongitudinal axis of the transmitting coil is 15-100 microns.

For some applications, a height of the flexible PCB measured along alongitudinal axis of the transmitting coil is 100-200 microns.

For some applications, respective wires of the two layers areconductively connected to each other at at least one location along eachturn of the transmitting coil.

For some applications, the apparatus further includes at least onecapacitor, coupled to the transmitting coil at at least one locationalong at least one turn of the transmitting coil.

For some applications, the capacitor is electrically coupled to both ofthe two layers.

For some applications, the apparatus further includes a plurality ofcapacitors coupled to the transmitting coil such that at least onecapacitor is coupled to the transmitting coil at at least one locationalong each turn of the transmitting coil.

For some applications, each of the capacitors is electrically coupled toboth of the two layers.

For some applications, an insulating cover is coupled to both layers ofthe transmitting coil disposed on the flexible PCB.

For some applications, an average distance from a wire of thetransmitting coil to the central longitudinal axis of the transmittingcoil is less than two times a square root of a cross-sectional area of acentral non-coiled region of the transmitting coil.

For some applications, the average distance from the wire of thetransmitting coil to the central longitudinal axis of the transmittingcoil is 0.6-1.5 times the square root of the cross-sectional area of thecentral non-coiled region of the transmitting coil.

There is further provided, in accordance with some applications of thepresent invention, apparatus for use with a medical implant thatincludes a receiving coil, the apparatus including:

a flexible housing configured to be placed against skin of a subject;

a flexible transmitting coil disposed in the housing;

control circuitry configured to transmit power to the medical implant bydriving a current through the transmitting coil that induces an inducedcurrent in the receiving coil;

a sensor coupled to the control circuitry, the sensor configured todetermine an extent of divergence of (a) a resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) anominal resonance frequency of the transmitting coil, occurring in theabsence of any forces applied to the transmitting coil, and configuredto output a signal according to the determination; and

one or more electrical components, coupled to the control circuitry andconfigured to tune the resonance frequency of the transmitting coil inresponse to the determination of the sensor.

For some applications, the control country is configured to set thefrequency of the current output by the control circuitry to be between 1and 20 MHz.

For some applications, the flexible transmitting coil is configured toflex such that it can substantially conform to a lateral wall of acylinder having a diameter between 8 and 50 cm.

For some applications, the sensor includes a phase detector, configuredto (a) determine a phase difference between the phase of the currentoutput by the control circuitry, and the phase of either a current or avoltage on at least, one component, of the transmitting coil, the phasedifference being due to flexing of the transmitting coil, and (b) outputa signal according to the determination.

For some applications, the control circuitry includes a feedbackcalculator configured to:

(a) receive the signal output by the phase detector,

(b) determine, according to the signal output by the phase detector, anecessary change in the resonance frequency of the transmitting coil, inorder to reduce the extent of divergence of (a) the resonance frequencyof the transmitting coil when the transmitting coil is flexed from (b)the nominal resonance frequency of the transmitting coil, and

(c) output a signal to the one or more electrical components, accordingto the determination.

For some applications, the sensor is configured to:

(a) measure a parameter that is indicative of the frequency of thecurrent, output by the control circuitry and the resonance frequency ofthe transmitting coil,

(b) look up at least one value in a look-up table with respect to themeasured parameter, and

(c) output a signal to the one or more electrical components based onthe looked-up value.

For some applications, the control circuitry is configured such that themeasured parameter is a level of power output by the transmitting coil.

For some applications, at least one of the one or more electricalcomponents is a variable inductor, the control circuitry is configuredto vary an inductance of the variable inductor according to the signaloutput by the sensor, and the resonance frequency of the transmittingcoil varies according to the variation of the inductance of the variableinductor.

For some applications, at least one of the one or more electricalcomponents is a variable capacitor, the control circuitry is configuredto vary a capacitance of the variable capacitor according to the signaloutput, by the sensor, and the resonance frequency of the transmittingcoil, varies according to the variation of the capacitance of thevariable capacitor.

For some applications, the apparatus further includes a plurality ofswitches, each switch coupled to a respective one of the electricalcomponents.

For some applications, the control circuitry is configured to tune theresonance frequency of the transmitting coil, according to the signaloutput by the sensor, by activating at least one of the plurality ofswitches to facilitate or inhibit current flow through the respectiveelectrical component.

For some applications, the control circuitry is configured to dither theresonance frequency of the transmitting coil by repeatedly activatingand deactivating the at least one of the plurality of switches tofacilitate or inhibit current flow through the respective electricalcomponent.

For some applications, at least one of the plurality of switches isconfigured to be manually operated and the remaining switches areconfigured to be operated by the control circuitry, wherein (a) theelectrical component coupled to the manually- operated switch isconfigured to vary the resonance frequency of the transmitting coil bymore than (b) the electrical components coupled to the switches operatedby the control circuitry are configured to vary the resonance frequencyof the transmitting coil.

For some applications, the one or more electrical components is aplurality of inductors, coupled in series.

For some applications, the plurality of inductors includes 3-9inductors.

For some applications, a first one of the inductors has an inductance of1.5-2.5 times an inductance of another one of the inductors.

For some applications, the inductance of the first one of the inductorsis twice the inductance of the other one of the inductors.

For some applications, each one of at least half of the inductors has aninductance which is twice an inductance of another one of the inductors.

For some applications, the control circuitry is configured such thatwhen the extent of divergence of (a) the resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) thenominal resonance frequency of the transmitting coil is reduced, currentis allowed to pass through at least one of the inductors and current isinhibited from passing through at least another one of the inductors.

For some applications, the one or more electrical components is aplurality of capacitors coupled in parallel.

For some applications, the plurality of capacitors includes 4 to 10capacitors.

For some applications, a first one of the capacitors has a capacitanceof 1.5-2.5 times a capacitance of another one of the capacitors.

For some applications, the capacitance of the first one of thecapacitors is twice the capacitance of the other one of the capacitors.

For some applications, each one of at least half of the capacitors has acapacitance that is twice a capacitance of another one of thecapacitors.

For some applications, the control circuitry is configured such thatwhen the extent of divergence of (a) the resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) thenominal resonance frequency of the transmitting coil is reduced, currentis allowed to pass through at least one of the capacitors and current isinhibited from passing through at least another one of the capacitors.

For some applications, the one or more electrical components is aplurality of electrical components including inductors, coupled inseries, and capacitors, coupled in parallel.

For some applications, a first one of the inductors has an inductance of1.5-2.5 times an inductance of another one of the inductors.

For some applications, the inductance of the first one of the inductorsis twice the inductance of the other one of the inductors.

For some applications, each one of at least half of the inductors has aninductance that is twice an inductance of another one of the inductors.

For some applications, a first one of the capacitors has a capacitanceof 1.5-2.5 times a capacitance of another one of the capacitors.

For some applications, the capacitance of the first one of thecapacitors is twice the capacitance of the other one of the capacitors.

For some applications, each one of at least half of the capacitors has acapacitance that is twice a capacitance of another one of thecapacitors.

For some applications, the control circuitry is configured such thatwhen the extent of divergence of (a) the resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) thenominal resonance frequency of the transmitting coil is reduced, currentis allowed to pass through at least one of the electrical components andcurrent is inhibited from passing through at least another one of theelectrical components.

For some applications:

the control circuitry is configured to activate the switches by applyinga respective voltage of 30-300 volts to each switch,

the switches include transistors, acting as diodes, having respectivecapacitances that are dependent on the respective voltage applied toeach switch.

For some applications, the control circuity is configured to apply therespective voltages to the respective switches at a voltage of 50-200volts.

For some applications:

the control circuitry is configured to activate the switches by applyinga respective voltage of 30-300 volts to each switch, and

the switches include transistors, which behave in their off states asvariable capacitors, having respective capacitances that are dependenton the respective voltage applied to each switch.

For some applications, the control circuity is configured to apply therespective voltages to the respective switches at a voltage of 50-200volts.

For some applications, the apparatus further includes the medicalimplant.

There is further provided, in accordance with some applications of thepresent invention, a method for treating a subject suffering frommigraines or cluster headaches, the method including:

identifying the subject, as suffering from migraines or clusterheadaches; and

in response to the identifying, powering a medical implant to stimulatea tibial nerve in a leg of the subject.

The present invention will be more fully understood from the followingdetailed description of applications thereof, taken together with thedrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B are schematic illustrations of a medical implant comprising areceiving coil under skin of a subject and a transmitting coil disposedin a housing that is placed against the skin, in accordance with someapplications of the present invention;

FIGS. 2A-B are schematic illustrations of a cross-sectional view of thereceiving coil disposed in the medical implant and the transmitting coildisposed in the housing against the skin, in accordance with someapplications of the present invention;

FIG. 3 is a schematic illustration of the orientation of thetransmitting coil with respect to the receiving coil, in accordance withsome applications of the present invention;

FIG. 4 is a schematic illustration of a top view and a cross-section ofthe transmitting coil, in accordance with some applications of thepresent invention;

FIGS. 5A-B are schematic illustrations of a cross-sectional view of thetransmitting coil and a top view of the transmitting coil, in accordancewith some applications of the present invention;

FIGS. 6-7 are schematic illustrations of the transmitting coil in thehousing being flexed to conform to a curve of a limb of the subject, inaccordance with some applications of the present invention;

FIGS. 8-13 are schematic illustrations of control circuitry of thetransmitting coil, in accordance with some applications of the presentinvention;

FIG. 14 is a circuit diagram of the control circuitry of thetransmitting coil, in accordance with some, applications of the presentinvention; and

FIG. 15 is a graph showing rate of change of capacitance versusdrain-to-source voltage change of a switch coupled to the controlcircuitry, in accordance with some applications of the presentinvention.

DETAILED DESCRIPTION

Reference is made to FIGS. 1A-B, which are schematic illustrations of atransmitting coil 20, disposed in a transmitting coil housing 22 that isplaced against skin 28 of a subject, and a medical implant 23, underskin 28 of a limb 30 of a subject, comprising a receiving coil 24 thatis disposed in a receiving coil housing 26, in accordance with someapplications of the present invention. Typically, receiving coil housing26 is oriented such that a central longitudinal axis 32 of receivingcoil 24 is substantially parallel to skin 28. Transmitting coil housing22 of transmitting coil 20 is typically placed against skin 28 andoriented such that a central longitudinal axis 34 (FIG. 2) oftransmitting coil 20 is substantially perpendicular to skin 28. Power istransmitted to medical implant 23 by activating control circuitry 36(FIG. 1B), coupled to transmitting coil housing 22, to drive a currentthrough transmitting coil 20, for example at a frequency of 1-20 MHz,e.g., a fixed frequency of 6.78 or 13.56 MHz. For some applications,lower frequencies such as 0.1-0.5 MHz may also be used. A magneticfield, for example magnetic field 52 (FIG. 2A), generated by the currentin transmitting coil 20, induces an induced current in receiving coil24.

As used in the present application, including in the claims, a “centrallongitudinal axis” of an elongate structure is the set of all centroidsof transverse cross-sectional sections of the structure along thestructure. Thus, the cross-sectional sections are locally perpendicularto the central longitudinal axis, which runs along the structure. (Ifthe structure is circular in cross-section, the centroids correspondwith the centers of the circular cross-sectional sections.)

As used in the present application, including in the claims,substantially parallel elements are to be understood as having an anglebetween them that is less than 10 degrees. For some applications,substantially parallel elements have an angle between them that is lessthan 5 degrees.

As used in the present application, including in the claims,substantially perpendicular elements are to be understood as having anangle between them that is at least 85 degrees and/or less than 95degrees.

Reference is now made to FIGS. 2A-B, which are schematic illustrationsof an orientation of transmitting coil housing 22, and therebytransmitting coil 20, with respect to receiving coil 24, in accordancewith some applications of the present invention. For some applications,medical implant 23 is implanted between a knee and an ankle of asubject, closer to the ankle than to the knee, such as is shown in FIGS.1A-B. Typically, a doctor will implant the medical implant 2-10 cm awayfrom the medial malleolus. Inside medical implant 23, receiving coil 24is disposed closer to an ankle-side 34 (FIG. 2A) of the medical implantthan it is to a knee-side 86 (FIG. 2A) of the medical implant. Space onskin 23 near the ankle however is limited due to the subject's anklebone and shoe. Efficiency of the power transfer can be improved byensuring that magnetic fields, e.g., magnetic field 52, generated by thecurrent in transmitting coil 20 are substantially parallel to receivingcoil 24 in the vicinity of receiving coil 24. Magnetic fields that aresubstantially parallel to receiving coil 24, but not in the vicinity ofreceiving coil 24, e.g., magnetic field 53, do not substantially affectthe power transfer.

Therefore, taking into account the limited space available near theankle, the desired orientation of the generated magnetic fields isaccomplished by placing transmitting coil housing 22 against skin 28such that transmitting coil 20, having a wire disposed at all rotationallocations about central longitudinal axis 34, is not centered overreceiving coil 24. Rather, only a portion 50 (FIG. 1A) of transmittingcoil 20 is disposed over receiving coil 24, such that at one of therotational locations about central longitudinal axis 34 of transmittingcoil 20, a line 38 extending from the wire and substantially parallel tocentral longitudinal axis 34 of transmitting coil 20 intersectsreceiving coil 24, and at 180 degrees from the rotational location, aline 40 extending from the wire and substantially parallel to centrallongitudinal axis 34 of transmitting coil 20 does not intersectreceiving coil 24. This orientation of transmitting coil 20, furtherdescribed hereinbelow, allows for the use of only one transmitting coil20 to power medical implant 23. Typically, (a) portion 50 oftransmitting coil 20, that is disposed over receiving coil 24, is closerto the ankle than (b) a portion 51 of transmitting coil 20, that is notdisposed over receiving coil 24, e.g., disposed at 180 degrees fromportion 50, is to the ankle (for example as shown in FIG. 1A).

For some applications, medical implant 23 is implanted on a leg betweenthe knee and the ankle, as described hereinabove, in order to treatpatients suffering from migraines or cluster headaches using tibialnerve stimulation. Transmitting coil 20 powers medical implant 23 inorder to provide neural stimulation to the tibial nerve, for example ata repetition rate of 10-60 Hz. Similarly to over-stimulation of theulnar nerve for treatment of migraines, over-stimulation of the tibialnerve may cause paresthesia in the active pain centers in the brain,thereby reducing the pain of the migraine or cluster headache.

Reference is now made to FIG. 3, which is a schematic illustration oftransmitting coil 20 disposed in transmitting coil housing 22 andreceiving coil 24 disposed in receiving coil housing 26, in accordancewith some applications of the present invention. Typically, transmittingcoil housing 22 is placed such that (a) a first distance D1, fromcentral longitudinal axis 34 of transmitting coil 20 to a longitudinalcenter 42 of receiving coil 24, is 15-45 mm, (b) a second distance D2,from central longitudinal axis 34 of transmitting coil 20 to an inneredge 44 of portion 50 of transmitting coil 20 that is disposed overreceiving coil 24, is less than 30 mm, and (c) a third distance D3, fromcentral longitudinal axis 34 of transmitting coil 20 to an outer edge 46of portion 50 of transmitting coil 20 that is disposed over receivingcoil 24, is 40-60 mm. Typically, (a) first distance D1 is greater thansecond distance D2 and less than third distance D3, and (b) a differencebetween second distance D2 and third distance D3 is 30-40 mm. Thedifference between third distance D3 and second distance D2 is referredto hereinbelow as width W of transmitting coil 20.

In order to further improve the efficiency of the power transfer,transmitting coil 20 is typically elongated in a direction perpendicularto central longitudinal axis 32 of receiving coil 24 thus increasing adistance between central longitudinal axis 34 and a wire of transmittingcoil 20. Therefore, magnetic fields generated by the current intransmitting coil 20, e.g., magnetic field 54, that are notsubstantially parallel to receiving coil 24, are farther away fromreceiving coil 24 thereby they have less of an effect on the inducedcurrent in receiving coil 24. For some applications, an average distanceD10 (FIG. 4) from the wire of transmitting coil 20 to centrallongitudinal axis 34 of transmitting coil 20 is less than two times,e.g., 0.6-1.5 times, a square root of a cross-sectional area of acentral non-coiled region 56 of transmitting coil 20.

Efficiency of the power transfer is also affected by a depth ofimplantation of medical implant 23. Typically, medical implant 23 isimplanted at a depth D14 (FIG. 2B) of 1-5 cm below skin 28. As used inthe present application, including in the claims, the depth of medicalimplant 23 is the distance from skin 28 to central longitudinal axis 32of receiving coil 24 measured substantially normal to the skin.

Reference is again made to FIG. 1B. In some applications, an indicator48 is coupled to transmitting coil housing 22. Control circuitry 36 isconfigured to activate indicator 48 upon transmitting coil 20 being inan acceptable position with respect to receiving coil 24. For example,indicator 48 may be a visual indicator, an audible indicator, or avibrator. Typically, indicator 48 is configured to indicate anacceptable position of transmitting coil 20 when the efficiency of theenergy transmission between transmitting coil 20 and receiving coil 24is above a threshold that is at least 85% of the maximum efficiencypossible for the patient. For some applications, the maximum efficiencyof the power transfer is approximately 5%.

For some applications, control circuitry 36 is able to detect whentransmitting coil 20 is in an acceptable position by outputting a signaland subsequently detecting an interference, caused by receiving coil 24,with the signal. Upon detection of the interference, control circuitry36 activates indicator 48.

Alternatively or additionally, control circuitry 36 is able to ascertainan indication of the efficiency of the energy transmission betweentransmitting coil 20 and receiving coil 24, and indicator 48 isconfigured to have a range of indications that are respectivelyrepresentative of the efficiency ascertained by control circuitry 36.For some applications, the indication of the efficiency is a measurementof power loss in transmitting coil 20. Power loss in transmitting coil20 may include one or more of the following: (a) power losses that donot appreciably change with the positioning of the transmitting coil,such as losses due to unavoidable resistance of transmitting coil 20 andother losses in the transmitting electronics, and (b) losses in thepower transmitted to medical implant 23 which depend on the relativepositioning of transmitting coil 20 and receiving coil 24, such asabsorption of power in the tissue and surrounding structures. Thus,monitoring the power loss in transmitting coil 20 may facilitate properpositioning of transmitting coil 20 in relation to medical implant 23.

Alternatively or additionally, medical implant 23 is configured to sendan output signal to control circuitry 36 upon receiving transmittedpower from transmitting coil 20. This output signal may include dataindicative of the power received by receiving coil 24 in medical implant23. Control circuitry 36 receives the data indicative of the powerreceived by receiving coil 24 in medical implant 23, and by comparing itto the power transmitted by transmitting coil 20, determines a parameterindicative of the efficiency of the power transmission. This parametermay be used to indicate to the user: a) if the efficiency is within arange of acceptable values; and b) if repositioning transmitting coilhousing 22 has caused an increase or decrease in the power transmission.The indication may be used by a healthcare provider, during an initialtraining session, to train the patient or family member to correctlyposition transmitting coil housing 22. Similarly, the indication may beused by the patient or family member each time transmitting coil housing22 has to be placed on the patient or repositioned. For someapplications, the output signal from medical implant 23, indicative ofthe power received by receiving coil 24, is sent only when needed. Forexample, the output signal from medical implant 23 may be sent (a) whenmedical implant 23 is powered-up, (b) during positioning of transmittingcoil 20, or (c) when the power received by receiving coil 24 in medicalimplant 23 is changed unexpectedly, indicating a possible movement oftransmitting coil 20 relative to receiving coil 24. Transmitting coilhousing 22 can be positioned on skin 28 by placing housing 22 againstskin 28 and subsequently sliding transmitting coil housing 22 along skin28 until indicator 48 indicates that transmitting coil 20 is in anacceptable position with respect to receiving coil 24. In someapplications, control circuitry 36 is further configured to activateindicator 48 again upon transmitting coil 20 no longer being in anacceptable position with respect to receiving coil 24.

Reference is now made to FIG. 4, which is a schematic illustration oftransmitting coil 20 in accordance with some applications of the presentinvention. Typically, transmitting coil 20 is a planar coil havingbetween 4 and 10 turns, e.g., 8 turns. A line spacing D9 of adjacentcoplanar wires in transmitting coil 20 is typically 0.25-3 mm, e.g., 2mm, and a line width D8 of the wires in transmitting coil 20 istypically 1-4 mm, e.g., 2 mm. For some applications, transmitting coil20 comprises a plurality of planar layers, e.g., two planar layers.

One or more dimensions of transmitting coil 20 that highlight the planarproperties of transmitting coil 20 are as follows:

-   -   a height D5 (FIG. 2B) of transmitting coil 20, measured along        central longitudinal axis 34 of transmitting coil 20 when        transmitting coil 20 is laid flat, i.e. the thickness of        transmitting coil 20, is at least 300 and/or less than 600        microns;    -   an outer diameter D6 (FIG. 3) of transmitting coil 20 is at        least 100 mm and/or less than 140 mm;    -   a ratio of outer diameter D6 of transmitting coil 20 to height        D5 of transmitting coil 20 is at least 150.

As used in the present application, including in the claims, outerdiameter D6 of transmitting coil 20 is the largest dimension oftransmitting coil 20 from one side of the coil to the other, measuredperpendicular to central longitudinal axis 34 of transmitting coil 20.

Typically, a cross-sectional area 108 of the wire of transmitting coil20 is rectangular when the cross-section, e.g., cross-section A-A shownin FIG. 4, is taken perpendicular to a direction of current flow withinthe wire.

Typically, receiving coil 24 is a cylindrical coil having 10-40 turns,e.g., 20 turns, and comprising a ferrite core. For some applications,one or more dimensions of receiving coil 24 are as follows:

-   -   a longitudinal length D4 (FIG. 3) of receiving coil 24 is at        least 3 mm and/or less than 15 mm;    -   an outer diameter D7 of receiving coil 24 (FIG. 2B) is at least        0.6 mm and/or less than 1.5 mm; and/or    -   a ratio of outer diameter D7 of receiving coil 24 to        longitudinal length D4 of receiving coil 24 is less than 0.5.

Typically, receiving coil housing 26 is longitudinally longer thanreceiving coil 24, to accommodate for control circuitry disposed withinmedical implant 23. For some applications, a longitudinal length D11 ofreceiving coil housing 26 is at least 30 mm and/or less than 45 mm.Medical implant 23 may also comprise a plurality of electrodes.

For some applications, some dimensional relationships betweentransmitting coil 20 and receiving coil 24 are expressed according to aset of one or more of the following options:

-   -   (a) a first ratio, of outer diameter D6 (FIG. 3) of transmitting        coil 20 to height D5 (FIG. 2B) of transmitting coil 20 is at        least 150, (b) a second ratio, of outer diameter D1 (FIG. 2B) of        receiving coil 24 to longitudinal length D4 (FIG. 3) of        receiving coil 24 is less than 0.5, and (c) a ratio of the first        ratio to the second ratio is at least 300;    -   a ratio of width W of transmitting coil 20 to longitudinal        length D4 (FIG. 3) of receiving coil 24 is greater than 0.5;    -   a ratio of width W of transmitting coil 20 to longitudinal        length D4 of receiving coil 24 is less than 1.5; and/or    -   a ratio of width W of transmitting coil 20 to longitudinal        length D4 of receiving coil 24 is at least 0.5 and/or less than        1.5.

Reference is now made to FIGS. 5A-B, which are schematic illustrationsof transmitting coil 20, in accordance with some applications of thepresent invention. FIG. 5A depicts schematic cross-sectional views ofseveral locations of transmitting coil 20, in accordance with someapplications of the present invention. For some applications,transmitting coil 20 comprises two planar layers 94 and S6 disposed oneither side of a flexible printed circuit board (PCB) 98. A height D12of each planar layer 94 and 96, measured along longitudinal axis 34 oftransmitting coil 20, is 15-100 microns, e.g., 35 or 70 microns, and athickness D13 of flexible PCB 98 is 100-200 microns, e.g., 150 microns.For some applications, at least once along each turn of transmittingcoil 20 the two planar layers 94 and 96 are conductively connected toeach other, such that current may flow from one layer to the other. Forexample, a via 100 filled with solder may be used to conductivelyconnect the two planar layers 94 and 96.

Additionally, a capacitor 102 is coupled to transmitting coil 20 at atleast one location along at least one turn of transmitting coil 20.Typically, capacitor 102 is attached to an exposed pad 92 of one ofplanar layers 94 or 96. For some applications, as seen in FIG. 5A,capacitor 102 is electrically coupled to both planar layers 94 and 96 bybeing coupled to two or more vias 100 in pad 92. For some applications,as seen in FIG. 5B, capacitor 102 is directly soldered to pad 92. Forsome applications, a plurality of capacitors 102 are coupled totransmitting coil 20 such that at least, one capacitor 102 is coupled totransmitting coil 20 at at least one location along each turn oftransmitting coil 20.

Typically, an Insulating cover 104 is coupled, e.g., glued, to bothplanar layers 94 and 96 of transmitting coil 20 on flexible PCB 98. Forsome applications, a thickness D17 of a layer of glue 106 between cover104 and each planar layer 94 and 96 is 15-50 microns. For someapplications, a thickness D18 of cover 104 is 15-100 microns.

FIG. 5A shows both a cross-section and a top-view of transmitting coil20. In the top-view, one planar layer 94 can be seen on flexible PCB 98,with one capacitor 102 coupled to each turn of transmitting coil 20. Aplurality of solder-filled vias 100 are coupled to each turn oftransmitting coil 20 to conductively connect planar layer 94 to planarlayer 96, which is coupled to the other side of flexible PCB 98 and notvisible in this figure. For some applications, vias 100 are positionedat the corners of each turn of transmitting coil 20, and on either sideof each capacitor 102, as shown in FIGS. 5A. Alternatively oradditionally a plurality of vias 100, e.g., 2-30 vias 100 may bepositioned anywhere along each turn of transmitting coil 20.

Reference is now made to FIGS. 6-7, which are schematic illustrations oftransmitting coil housing 22, comprising transmitting coil 20, placedagainst skin 23 of limb 30 of the subject, in accordance with someapplications of the present invention. Typically, in order to allowcomfortable placement of transmitting housing 22 against limb 30,transmitting coil housing 22 and transmitting coil 20 are configured tobe flexible such that they can substantially conform to a lateral wallof cylinders having diameters that range between a diameter D15 (FIG. 6)of 8 cm, e.g., a wrist, and a diameter D16 (FIG. 7) of 50 cm, e.g., atorso or obese upper leg. The flexing of transmitting coil 20, however,may cause the resonance frequency of transmitting coil 20 to fluctuaterather than remain at a nominal resonance frequency that occurs in theabsence of any forces applied to transmitting coil 20 and is near thefrequency of the current output by control circuitry 36.

Reference is now made to FIGS. 8-13, which are circuit diagrams ofcontrol circuitry 36, in accordance with some applications of thepresent invention. Portion 21 of control circuitry 36, as shown in FIGS.8-13, is a model of transmitting coil 20 as shown in FIGS. 1-7. For someapplications, a sensor 58 is coupled to control circuitry 36. Sensor 58is coupled to control circuitry 36 and is configured to (a) determine anextent of divergence of (i) the resonance frequency of transmitting coil20 when transmitting coil 20 is flexed from (ii) the nominal resonancefrequency of transmitting coil 20, and (b) subsequently output a signalto one or more electrical components that are coupled to controlcircuitry 36 and configured to tune the resonance frequency oftransmitting coil 20 in response to the determination of sensor 58.

For some applications, sensor 58 comprises a phase detector 60 and afeedback calculator 62 (for example, as shown in FIGS. 8-12). Phasedetector 60 is configured to (a) determine a phase difference betweenthe phase of the current output by signal generator 64 and the phase ofeither a current or a voltage on at least one component: of transmittingcoil 20, and (b) output a signal to feedback calculator 62 according tothe determination. After receiving the signal from phase detector 60,feedback calculator 62 (a) determines a necessary change in theresonance frequency of transmitting coil 20 that will reduce the extentof divergence of (i) the resonance frequency of transmitting coil 20when transmitting coil 20 is flexed from (ii) the nominal resonancefrequency of transmitting coil 20 and (b) outputs a signal to theelectrical components according to the determination. Dashed lines 88(FIGS. 8-12 represent feedback calculator 62 controlling each respectiveswitch 74.

For some applications (e.g., as shown in FIG. 13), sensor 58 does notcomprise phase detector 60 and feedback calculator 62. Rather, sensor 58is configured to (a) measure a parameter that is indicative of both thefrequency output by signal generator 64 and the resonance frequency oftransmitting coil 20, e.g., by measuring the power output oftransmitting coil 20, (b) look up at least one value in a look-up tablewith respect to the measured parameter, and (c) output a signal to theelectrical components based on the looked-up value. Dashed line 90 (FIG.13) represents sensor 58 controlling each respective switch 74.

For some applications, at least one of the electrical components is avariable inductor 66 (FIG. 8), whose inductance is varied according tothe signal output by sensor 58, Variation of the inductance of variableinductor 66, in turn, cause variations in the resonance frequency oftransmitting coil 20.

For some applications, at least one of the electrical components is avariable capacitor 68 (FIG. 9), whose capacitance is varied according tothe signal output by sensor 58. Variation of the capacitance of variablecapacitor 68, in turn, cause variations in the resonance frequency oftransmitting coil 20.

For some applications, the one or more electrical components is a (a) aplurality of inductors 70, e.g., 3-9 inductors 70, coupled in series(FIG. 10), (b) a plurality of capacitors 72, e.g., 4-10 capacitors 72,coupled in parallel (FIG. 21), or (c) a combination of inductors 70,coupled in series, and capacitors 72, coupled in parallel (FIG. 12).When a plurality of inductors 70 are used, typically a first one ofinductors 70 has an inductance of 1.5-2.5 times, e.g., 2 times, aninductance of another one of inductors 70, and/or each one of at leasthalf of inductors 70 has an inductance that is twice an inductance ofanother one of inductors 70. For example, 9 inductors 70 may haverespective inductances of 2, 4, 8, 16, 32, 64, 128, 256, and 512(arbitrary units). Similar sequencing may be used for a plurality ofcapacitors 72. For example, 10 capacitors 72 may have respectivecapacitances of 2, 4, 8, 16, 32, 64, 128, 256, 512, and 1024 (arbitraryunits).

Typically, control circuitry 36 tunes the resonance frequency oftransmitting coil 20, according to the signal output by sensor 58, byactivating and/or deactivating at least one of a plurality of switches74, each switch 74 being coupled to a respective one of the electricalcomponents, in order to facilitate or inhibit current flow through therespective electrical component. In order to easily be able to increaseand decrease the resonance frequency of transmitting coil 20, asnecessary according to the signal output by sensor 58, control circuitry36 is configured such that, when the extent of divergence of (a) theresonance frequency of transmitting coil 20 when transmitting coil 20 isflexed from (b) the nominal resonance frequency of transmitting coil 20is reduced, at least one of switches 74 is activated, allowing currentto flow through a respective electrical component, and at least anotherswitch 74 is deactivated, inhibiting current from flowing throughanother respective electrical component. For some applications, controlcircuitry 36 is configured to dither the resonance frequency oftransmitting coil 20 by repeatedly activating and deactivating at leastone of switches 74 to alternatingly facilitate and inhibit current flowthrough a respective electrical component.

For some applications, a wider range of variation of the resonancefrequency of transmitting coil 20 may be achieved by having at least oneelectrical component, (a) configured to vary the resonance frequency oftransmitting coil 20 by more than the remaining electrical componentsare configured to vary the resonance frequency of transmitting coil 20and (b) coupled to a manually-operated switch. The manually-operatedswitch may be activated and/or deactivated by a user to provide grosstuning of the resonance frequency of transmitting coil 20 and theremaining switches 74 activated and/or deactivated by control circuitry36 to provide fine tuning of the resonance frequency of transmittingcoil 20.

Reference is now made to FIG. 14, which is a circuit diagram of controlcircuitry 36, in accordance with some applications of the presentinvention. For some applications, switches 74 comprise transistors 76that behave, in their off state, as either diodes or variablecapacitors, such that each switch 74 has a respective parasiticcapacitance that depends on a respective voltage applied to each switch74.

Reference is now made to FIG. 15, which is a graph showing rate ofcapacitance change versus drain-to-source voltage change of a switch,such as a switch 74 in control circuitry 36, in accordance with someapplications of the present invention. Curve 78 of the graph shows (a)how the rate of capacitance change of a switch, such as switch 74, issignificantly decreased when the switch is activated by an alternatingcurrent (AC) voltage of over 50 volts, and (b) how the outputcapacitance of a switch, such as switch 74, significantly decreases asthe drain-to-source voltage is increased from 0-50 volts. As shown byarrows 80 and 82, in order to reduce an effect that the respectiveparasitic capacitances of respective switches 74 may have on theresonance frequency of transmitting coil 20, control circuitry 36 isconfigured to activate switches 74 by applying a respective AC voltageof 30-300 volts (arrow 80), e.g., 50-200 volts (arrow 82), to eachswitch 74, thereby reducing the output capacitance of each switch 74, aswell as reducing the variation in output capacitance of each switch 74over the duration of the AC voltage cycle.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1. A method for transmitting power to a medical implant that comprises areceiving coil that is oriented such that a longitudinal axis of thereceiving coil is substantially parallel to skin of a subject, themethod comprising: providing a transmitting coil disposed in a housing;placing the housing against the skin such that: (a) a centrallongitudinal axis of the transmitting coil is substantiallyperpendicular to the skin, (b) a portion of the transmitting coil isdisposed over the receiving coil, (c) a first distance, from the centrallongitudinal axis of the transmitting coil to a longitudinal center ofthe receiving coil, is greater than a second distance, from the centrallongitudinal axis of the transmitting coil to an inner edge of theportion of the transmitting coil, and (d) the first distance is lessthan a third distance, from the central longitudinal axis of thetransmitting coil to an outer edge of the portion of the transmittingcoil; and activating control circuitry to power the medical implant bydriving a current through the transmitting coil that induces an inducedcurrent in the receiving coil.
 2. The method according to claim 1,wherein placing comprises identifying the subject as suffering frommigraines or cluster headaches, and in response to the identifying,placing the housing on a leg of a subject such that: (a) thetransmitting coil is disposed between a knee and an angle of the leg,and (b) the transmitting coil transmits power to a medical implantconfigured to stimulate a tibial nerve in the leg of the subject.
 3. Themethod according to claim 1, wherein placing comprises placing thehousing on a leg of the subject such that: (a) the transmitting coil isdisposed between a knee and an ankle of the leg, and (b) (i) a portionof the transmitting coil that is disposed over the receiving coil iscloser to the ankle than (ii) a portion of the transmitting coil that isdisposed at 180 degrees from the portion of the transmitting coil thatis disposed over the receiving coil, is to the ankle.
 4. The methodaccording to claim 1, wherein placing comprises placing the housing suchthat the first distance is 15-45 mm.
 5. The method according to claim 1,wherein placing comprises placing the housing such that, the seconddistance is less than 30 mm.
 6. The method according to claim 1, whereinplacing comprises placing the housing such that the third distance is40-60 mm.
 7. The method according to claim 1, wherein placing comprisesplacing the housing such that a difference between the third distanceand the second distance is 30-40 mm.
 8. The method according to claim 1,wherein providing the transmitting coil comprises providing atransmitting coil wherein a ratio of (a) a difference between the thirddistance and the second distance, to (b) a longitudinal length of thereceiving coil is greater than 0.5.
 9. The method according to claim 1,wherein providing the transmitting coil comprises providing atransmitting coil wherein a ratio of (a) a difference between the thirddistance and the second distance, to (b) a longitudinal length of thereceiving coil is less than 1.5.
 10. The method according to claim 1,wherein providing the transmitting coil, comprises providing atransmitting coil wherein a ratio of (a) a difference between the thirddistance and the second distance, to (b) a longitudinal length of thereceiving coil is between 0.5 and 1.5.
 11. The method according to claim1, wherein providing the transmitting coil comprises providing atransmitting coil wherein: (a) a height of the transmitting coilmeasured along a longitudinal axis of the transmitting coil is 300-600microns, (b) an outer diameter of the transmitting coil is 100-140 mm,and (c) a ratio of the outer diameter of the transmitting coil to theheight of the transmitting coil is at least
 150. 12. The methodaccording to claim 1, wherein placing comprises placing the housing suchthat the transmitting coil is over a receiving coil, wherein: (a) alongitudinal length of the receiving coil is 3-15 mm, (b) an outerdiameter of the receiving coil is 0.6-1.5 mm, and (c) a ratio of theouter diameter of the receiving coil to the longitudinal length of thereceiving coil is less than 0.5.
 13. The method according to claim 1,wherein activating the control circuitry comprises activating thecontrol circuitry to drive the current through the transmitting coil ata frequency of 1-20 MHz.
 14. The method according to claim 1, whereinplacing comprises placing the housing against the skin and subsequentlysliding it along the skin until an indicator, coupled to the housing,indicates that the transmitting coil is in an acceptable position withrespect to the receiving coil.
 15. The method according to claim 1,wherein providing the transmitting coil, comprises providing atransmitting coil wherein a cross-sectional area of a wire of thetransmitting coil is rectangular, wherein the cross-section is takenperpendicular to a direction of current flow within the wire.
 16. Themethod according to claim 1, wherein providing the transmitting coilcomprises providing a transmitting coil that is elongated in a directionperpendicular to the central longitudinal axis of the receiving coil.17. The method according to claim 1, wherein providing the transmittingcoil comprises providing a planar coil disposed in a housing.
 18. Themethod according to claim 17, wherein providing the planar coilcomprises providing a planar coil comprising a plurality of layers. 19.The method according to claim 18, wherein providing the planar coilcomprises providing a planar coil with a line spacing, of adjacentcoplanar wires, of 0.25-3 mm.
 20. The method according to claim 18,wherein providing the planar coil comprises providing a planar coil witha line width of 1-4 mm.
 21. The method according to any one of claims1-20, wherein providing the transmitting coil comprises providing atransmitting coil wherein an average distance from a wire of thetransmitting coil to the central longitudinal axis of the transmittingcoil is less than two times a square root of a cross-sectional area of acentral non-coiled region of the transmitting coil.
 22. The methodaccording to claim 21, wherein providing comprises providing atransmitting coil wherein an average distance from the wire of thetransmitting coil to the central longitudinal axis of the transmittingcoil is 0.6-1.5 times the square root of the cross-sectional area of thecentral non-coiled region of the transmitting coil.
 23. Apparatuscomprising: a medical implant, the medical implant comprising: areceiving coil; and a plurality of electrodes; a transmitting coil,having wire disposed at all rotational locations about a centrallongitudinal axis of the transmitting coil, oriented such that: (a) thecentral longitudinal axis of the transmitting coil is substantiallyperpendicular to a central longitudinal axis of the receiving coil, (b)at one of the rotational locations, a line from the wire andsubstantially parallel to the central longitudinal axis of thetransmitting coil intersects the receiving coil, and at 180 degrees fromthe rotational location a line from the wire and substantially parallelto the central longitudinal axis of the transmitting coil does notintersect the receiving coil, (c) a first, distance from the centrallongitudinal axis of the transmitting coil to a longitudinal center ofthe receiving coil, is greater than a second distance from the centrallongitudinal axis of the transmitting coil to an inner edge of thetransmitting coil at the one of the rotational locations, and (d) thefirst distance is less than a third distance from the centrallongitudinal axis of the transmitting coil to an outer edge of thetransmitting coil at the one of the rotational locations; and controlcircuitry configured to transmit power to the medical implant by drivinga current through the transmitting coil that induces an induced currentin the receiving coil.
 24. The apparatus according to claim 23, whereinthe control circuitry is configured to drive the current through thetransmitting coil at a frequency of 1-20 MHz.
 25. The apparatusaccording to claim 23, wherein the medical implant is configured to beimplanted 1-5 cm below skin of a subject, and wherein the controlcircuitry is configured to transmit the power, by driving the currentthrough the transmitting coil that induces the induced current in thereceiving coil, when the medical implant is implanted 1-5 cm below theskin.
 26. The apparatus according to claim 23, wherein the receivingcoil is a cylindrical coil comprising a ferrite core.
 27. The apparatusaccording to claim 23, wherein the first distance is 15-45 mm.
 28. Theapparatus according to claim 23, wherein the second distance is lessthan 30 mm.
 29. The apparatus according to claim 23, wherein the thirddistance is 40-60 mm.
 30. The apparatus according to claim 23, wherein adifference between the third distance and the second distance is 30-40mm.
 31. The apparatus according to claim 23, wherein a ratio of (a) adifference between the third distance and the second distance, to (b) alongitudinal length of the receiving coil is greater than 0.5.
 32. Theapparatus according to claim 23, wherein a ratio of (a) a differencebetween the third distance and the second distance, to (b) alongitudinal length of the receiving coil is less than 1.5.
 33. Theapparatus according to claim 23, wherein a ratio of (a) a differencebetween the third distance and the second distance, to (b) alongitudinal length of the receiving coil is between 0.5 and 1.5. 34.The apparatus according to claim 23, wherein: (a) a height of thetransmitting coil measured along a longitudinal axis of the transmittingcoil is 300-600 microns, (b) an outer diameter of the transmitting coilis 100-140 mm, and (c) a ratio of the outer diameter of the transmittingcoil to the height of the transmitting coil is at least
 150. 35. Theapparatus according to claim 23, wherein: (a) a longitudinal length ofthe receiving coil is 3-15 mm, (b) an outer diameter of the receivingcoil is 0.6-1.5 mm, and (c) a ratio of the outer diameter of thereceiving coil to the longitudinal length of the receiving coil is lessthan 0.5.
 36. The apparatus according to claim 23, wherein: (a) a firstratio, of the outer diameter of the transmitting coil to a height of thetransmitting coil measured along a longitudinal axis of the transmittingcoil, is at least 150, (b) a second ratio, of the outer diameter of thereceiving coil to the longitudinal length of the receiving coil, is lessthan 0.5, and (c) a ratio of the first ratio to the second ratio is atleast
 300. 37. The apparatus according to claim 23, wherein thetransmitting coil has between 4 and 10 turns.
 38. The apparatusaccording to claim 23, wherein the receiving coil has between 10 and 40turns.
 39. The apparatus according to claim 23, wherein the medicalimplant is configured to send a signal to the control circuitry uponreceiving the transmitted power.
 40. The apparatus according to claim23, wherein a cross-sectional area of a wire of the transmitting coil isrectangular, wherein the cross-section is taken perpendicular to adirection of current flow within the wire.
 41. The apparatus accordingto claim 23, wherein the transmitting coil is elongated in a directionperpendicular to the central longitudinal axis of the receiving coil.42. The apparatus according to claim 23, wherein a length of thereceiving coil is 3-15 mm.
 43. The apparatus according to claim 42,wherein the medical implant comprises a housing having a length of 30-45mm and wherein the receiving coil is disposed within the housing. 44.The apparatus according to any one of claims 23-43, further comprisingan indicator, and wherein the control circuitry is configured toactivate the indicator upon the transmitting coil being in an acceptableposition with respect to the receiving coil.
 45. The apparatus accordingto claim 44, wherein the control circuitry is configured to detectinterference with its output signal and to activate the indicator uponthe detection of the interference.
 46. The apparatus according to claim44, wherein the control circuitry is configured to activate theindicator again, upon the transmitting coil no longer being in correctposition with respect to the receiving coil.
 47. The apparatus accordingto claim 44, wherein the control circuitry is configured to ascertain anindication of an efficiency of the power transfer between thetransmitting coil and the receiving coil, and to activate the indicatoraccording to the ascertaining.
 48. The apparatus according to claim 47,wherein the control circuitry is configured to measure a loss of powerin the transmitting coil, the loss of power being indicative of theefficiency of the power transfer.
 49. The apparatus according to any oneof claims 23-43, wherein the transmitting coil is a planar coil.
 50. Theapparatus according to claim 49, wherein a line width of thetransmitting coil is 1-4 mm.
 51. The apparatus according to claim 49,wherein the planar coil comprises a plurality of layers.
 52. Theapparatus according to claim 51, wherein a line spacing of adjacentcoplanar wires in the transmitting coil is 0.25-3 mm.
 53. The apparatusaccording to claim 51, further comprising a flexible printed circuitboard (PCB), wherein the transmitting coil comprises two planar layersdisposed on either side of the flexible PCB.
 54. The apparatus accordingto claim 53, wherein a height of each layer measured along alongitudinal axis of the transmitting coil is 15-100 microns.
 55. Theapparatus according to claim 53, wherein a height of the flexible PCBmeasured along a longitudinal axis of the transmitting coil is 100-200microns.
 56. The apparatus according to claim 53, wherein respectivewires of the two layers are conductively connected to each other at atleast one location along each turn of the transmitting coil.
 57. Theapparatus according to claim 53, further comprising at least onecapacitor, coupled to the transmitting coil at at least one locationalong at least one turn of the transmitting coil.
 58. The apparatusaccording to claim 57, wherein the capacitor is electrically coupled toboth of the two layers.
 59. The apparatus according to claim 53, furthercomprising a plurality of capacitors coupled to the transmitting coilsuch that at least one capacitor is coupled to the transmitting coil atat least one location along each turn of the transmitting coil.
 60. Theapparatus according to claim 59 wherein the each of the capacitors iselectrically coupled to both of the two layers.
 61. The apparatusaccording to claim 53, wherein an insulating cover is coupled to bothlayers of the transmitting coil disposed on the flexible PCB.
 62. Theapparatus according to any one of claims 23-43, wherein an averagedistance from a wire of the transmitting coil to the centrallongitudinal axis of the transmitting coil is less than two times asquare root of a cross-sectional area of a central non-coiled region ofthe transmitting coil.
 63. The apparatus according to claim 62, whereinthe average distance from the wire of the transmitting coil to thecentral longitudinal axis of the transmitting coil is 0.6-1.5 times thesquare root of the cross-sectional area of the central non-coiled regionof the transmitting coil.
 64. Apparatus for use with a medical implantthat comprises a receiving coil, the apparatus comprising: a flexiblehousing configured to be placed against skin of a subject; a flexibletransmitting coil disposed in the housing; control circuitry configuredto transmit power to the medical implant by driving a current throughthe transmitting coil that induces an induced current in the receivingcoil; a sensor coupled to the control circuitry, the sensor configuredto determine an extent of divergence of (a) a resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) anominal resonance frequency of the transmitting coil, occurring in theabsence of any forces applied to the transmitting coil, and configuredto output a signal according to the determination; and one or moreelectrical components, coupled to the control circuitry and configuredto tune the resonance frequency of the transmitting coil in response tothe determination of the sensor.
 65. The apparatus according to claim64, wherein the control country is configured to set the frequency ofthe current output by the control circuitry to be between 1 and 20 MHz.66. The apparatus according to claim 64, wherein the flexibletransmitting coil is configured to flex such that it can substantiallyconform to a lateral wall of a cylinder having a diameter between 8 and50 cm.
 67. The apparatus according to any one of claims 64-66, whereinthe sensor comprises a phase detector, configured to (a) determine aphase difference between the phase of the current output by the controlcircuitry, and the phase of either a current or a voltage on at leastone component of the transmitting coil., wherein the phase difference isdue to flexing of the transmitting coil, and (b) output a signalaccording to the determination.
 68. The apparatus according to claim 67,wherein the control circuitry comprises a feedback calculator configuredto: (a) receive the signal output by the phase detector, (b) determine,according to the signal output by the phase detector, a necessary changein the resonance frequency of the transmitting coil, in order to reducethe extent of divergence of (a) the resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) thenominal resonance frequency of the transmitting coil, and (c) output asignal to the one or more electrical components, according to thedetermination.
 69. The apparatus according to any one of claims 64-66,wherein the sensor is configured to: (a) measure a parameter that isindicative of the frequency of the current output by the controlcircuitry and the resonance frequency of the transmitting coil, (b) lookup at least one value in a look-up table with respect to the measuredparameter, and (c) output a signal to the one or more electricalcomponents based on the looked-up value.
 70. The apparatus according toclaim 69, wherein the control circuitry is configured such that themeasured parameter is a level of power output by the transmitting coil.71. The apparatus according to any one of claims 64-66, wherein at leastone of the one or more electrical components is a variable inductor,wherein the control circuitry is configured to vary an inductance of thevariable inductor according to the signal output by the sensor, andwherein the resonance frequency of the transmitting coil variesaccording to the variation of the inductance of the variable inductor.72. The apparatus according to any one of claims 64-66, wherein at leastone of the one or more electrical components is a variable capacitor,wherein the control circuitry is configured to vary a capacitance of thevariable capacitor according to the signal output by the sensor, andwherein the resonance frequency of the transmitting coil variesaccording to the variation of the capacitance of the variable capacitor.73. The apparatus according to any one of claims 64-66, furthercomprising a plurality of switches, each switch coupled to a respectiveone of the electrical components.
 74. The apparatus according to claim73, wherein the control circuitry is configured to tune the resonancefrequency of the transmitting coil, according to the signal output bythe sensor, by activating at least one of the plurality of switches tofacilitate or inhibit current flow through the respective electricalcomponent.
 75. The apparatus according to claim 74, wherein the controlcircuitry is configured to dither the resonance frequency of thetransmitting coil by repeatedly activating and deactivating the at leastone of the plurality of switches to facilitate or inhibit current flowthrough the respective electrical component.
 76. The apparatus accordingto claim 73, wherein at least one of the plurality of switches isconfigured to be manually operated and the remaining switches areconfigured to be operated by the control circuitry, wherein (a) theelectrical component coupled to the manually-operated switch isconfigured to vary the resonance frequency of the transmitting coil bymore than (b) the electrical components coupled to the switches operatedby the control circuitry are configured to vary the resonance frequencyof the transmitting coil.
 77. The apparatus according to claim 73,wherein the one or more electrical components is a plurality ofinductors, coupled in series.
 78. The apparatus according to claim 77,wherein the plurality of inductors comprises 3-9 inductors.
 79. Theapparatus according to claim 77, wherein a first one of the inductorshas an inductance of 1.5-2.5 times an inductance of another one of theinductors.
 80. The apparatus according to claim 79, wherein theinductance of the first one of the inductors is twice the inductance ofthe other one of the inductors.
 81. The apparatus according to claim 77,wherein each one of at least half of the inductors has an inductancewhich is twice an inductance of another one of the inductors.
 82. Theapparatus according to claim 77, wherein the control circuitry isconfigured such that when the extent of divergence of (a) the resonancefrequency of the transmitting coil when the transmitting coil is flexedfrom (b) the nominal resonance frequency of the transmitting coil isreduced, current is allowed to pass through at least one of theinductors and current is inhibited from passing through at least anotherone of the inductors.
 83. The apparatus according to claim 73, whereinthe one or more electrical components is a plurality of capacitorscoupled in parallel.
 84. The apparatus according to claim 83, whereinthe plurality of capacitors comprises 4 to 10 capacitors.
 85. Theapparatus according to claim 83, wherein a first one of the capacitorshas a capacitance of 1.5-2.5 times a capacitance of another one of thecapacitors.
 86. The apparatus according to claim 85, wherein thecapacitance of the first one of the capacitors is twice the capacitanceof the other one of the capacitors.
 87. The apparatus according to claim83, wherein each one of at least half of the capacitors has acapacitance that is twice a capacitance of another one of thecapacitors.
 88. The apparatus according to claim 83, wherein the controlcircuitry is configured such that when the extent of divergence of (a)the resonance frequency of the transmitting coil when the transmittingcoil is flexed from (b) the nominal resonance frequency of thetransmitting coil is reduced, current is allowed to pass through atleast one of the capacitors and current is inhibited from passingthrough at least another one of the capacitors.
 89. The apparatusaccording to claim 73, wherein the one or more electrical components isa plurality of electrical components comprising inductors, coupled inseries, and capacitors, coupled in parallel.
 90. The apparatus accordingto claim 89, wherein a first one of the inductors has an inductance of1.5-2.5 times an inductance of another one of the inductors.
 91. Theapparatus according to claim 90, wherein the inductance of the first oneof the inductors is twice the inductance of the other one of theinductors.
 92. The apparatus according to claim 89, wherein each one ofat least half of the inductors has an inductance that is twice aninductance of another one of the inductors.
 93. The apparatus accordingto claim 89, wherein a first one of the capacitors has a capacitance of1.5-2.5 times a capacitance of another one of the capacitors.
 94. Theapparatus according to claim 93, wherein the capacitance of the firstone of the capacitors is twice the capacitance of the other one of thecapacitors.
 95. The apparatus according to claim 89, wherein each one ofat least half of the capacitors has a capacitance that is twice acapacitance of another one of the capacitors.
 96. The apparatusaccording to claim 89, wherein the control circuitry is configured suchthat when the extent of divergence of (a) the resonance frequency of thetransmitting coil when the transmitting coil is flexed from (b) thenominal resonance frequency of the transmitting coil is reduced, currentis allowed to pass through at least one of the electrical components andcurrent is inhibited from passing through at least another one of theelectrical components.
 97. The apparatus according to claim 73, wherein:the control circuitry is configured to activate the switches by applyinga respective voltage of 30-300 volts to each switch, the switchescomprise transistors, acting as diodes, having respective capacitancesthat are dependent on the respective voltage applied to each switch. 98.The apparatus according to claim 97, wherein the control circuity Isconfigured to apply the respective voltages to the respective switchesat a voltage of 50-200 volts.
 99. The apparatus according to claim 73,wherein: the control circuitry is configured to activate the switches byapplying a respective voltage of 30-300 volts to each switch, and theswitches comprise transistors, which behave in their off states asvariable capacitors, having respective capacitances that are dependenton the respective voltage applied to each switch.
 100. The apparatusaccording to claim 99, wherein the control circuity is configured toapply the respective voltages to the respective switches at a voltage of50-200 volts.
 101. The apparatus according to any one of claims 64-66,further comprising the medical implant.
 102. A method for treating asubject suffering from migraines or cluster headaches, the methodcomprising: identifying the subject as suffering from migraines orcluster headaches; and in response to the identifying, powering amedical implant to stimulate a tibial nerve in a leg of the subject.